[Agent Substrate] Enforce Source Discussion Closeout Gate in epic-resolution (#11328 Part 2)

What is the purpose of this issue?

Add a Source Discussion Closeout Gate to the epic-resolution skill to prevent Epic closure if upstream graduation criteria are unmet.

What is the context?

This is part 2 of 3 to close the "Discussion-graduation enforcement gap" identified in the #11328 audit. Even if epic-review ensures a mapping exists, the final closeout of the Epic must verify that the mapped upstream criteria were actually achieved. Currently, epic-resolution only checks Epic ACs.

Acceptance Criteria

• Update .agents/skills/epic-resolution/SKILL.md (and/or its references/epic-resolution-workflow.md audit file) to include a new Source Discussion Closeout Gate.
• Add the trigger condition: Fires when the Epic has a source Discussion or a Stage 2.5 mapping table.
• Require the epic-resolution workflow to reconcile upstream criteria before returning an Epic AC verdict.
• Enforce the failure mode: If any source Discussion criterion is not delivered, explicitly deferred, or converted into a follow-up ticket, the verdict MUST NOT be CLOSE_AS_COMPLETED.
• Define the valid alternative verdicts for this failure mode: e.g., CREATE_MISSING_SUBS, KEEP_OPEN, or CLOSE_AS_PARTIAL_WITH_FOLLOW_UPS.

Priority

• priority:high
• ai